Memorize This About QAM (Please)
by Leslie Ellis // March 17 2008
Making sure people in the cable television industry know QAM is an odd hobby of mine. It would be good if everyone in the business of digital TV transmission just knew it, cold. It is foundational. What the alphabet is to words, QAM is to shelf space on cable TV plant.
Yes this is a “spinach column.” Eat it. It’s good for you.
It is nearly impossible to have even a mildly technical discussion about a new cable product without bumping into QAM. The term is rife. Whether you’re talking HDTV channels, HDTV on demand, anything else on demand, DOCSIS 3, voice, switched digital video, or bandwidth in general, QAM is there.
Before we even unpack the acronym, know that QAM is tri-multaneously a technique, a thing, and a unit of measure. That’s why it borrows so many beats from the speed of thought when you hear it: It’s an unintended riddle.
Few other things have such varied characteristics as QAM. “What’s black and white and read all over” becomes “what’s a pizza box that shimmies that’s five gallons.” No wonder people glaze over.
QAM stands for Quadrature Amplitude Modulation. Modulation is a technique, used to turn spectrum into bandwidth. (“Spectrum comes from God, bandwidth comes from cash,” as Knology CTO Ricky Luke should get credit for saying.) People say it “kwahm,” like Mom, and “kwam,” like Spam.
QAM is exclusively digital. By contrast, think of the AM/FM radio in your. It’s analog. (You can tell by the volume of radio ads encouraging you to upgrade to digital HD radio.)
The “M” in “AM/FM” is for modulation. The “M” in QAM is for modulation. I’m (vastly) over-simplifying, but, QAM does for digital TV what AM/FM does for radio: It imprints what you want to see onto the plant, to get it to you.
So that’ the technique part.
The box part is the flourishing marketplace for the metal boxes called QAMs, which are required for every digital service currently riding on cable systems. Lots of companies make them; pricing is expected to commoditize.
The wow-factor in QAM hardware tends to be the density — how many digital channels can be packed into one box. Average is 24. Good is 48. Some vendors (with H names) are talking about “hecto-QAMs,” where “hecto” means 100.
The unit of measurement part goes like this: One QAM equals one analog (basic) channel, equals 6 MHz of spectrum, equals ten to 12 standard-definition video streams, equals two to three high definition video streams, equals 38.8 Mbps of broadband data, equals thousands of voice conversations. (For purists, that’s one 256-QAM.)
This is the part it’d be good for you to know, cold. Memorize it and keep it handy. It is the fundamental math of digital, and it tends to come up a LOT. It’s how operators reckon how much shelf space exists for new HD channels, or any other digital service.
Know this and thrive!
This column originally ran in the Platforms section of Multichannel News.